#include <sourcemod>

#pragma semicolon 1
#pragma newdecls required

public Plugin myinfo = {
	name        = "",
	author      = "",
	description = "",
	version     = "0.0.0",
	url         = ""
};

public void OnPluginStart()
{
	PrintToServer("Hello, World!");
	OperateIndirictly(Add, 1, 2);
}

typedef OperateInts = function int (int i_a, int i_b);

int Add(int i_a, int i_b)
{
	return i_a + i_b;
}

int OperateIndirictly(OperateInts func, int i_a, int i_b)
{
	return func(i_a, i_b);
}

For some reason this throws that:

SourceMod	AMX Mod X
Light	Dark
Full Screen
SourcePawn Compiler 1.9.0.5921
Copyright (c) 1997-2006 ITB CompuPhase
Copyright (c) 2004-2015 AlliedModders LLC

plugin.sp(17) : error 100: function prototypes do not match
plugin.sp(29) : error 012: invalid function call, not a valid address
plugin.sp(29) : warning 215: expression has no effect
plugin.sp(29) : warning 215: expression has no effect
plugin.sp(29) : error 001: expected token: ";", but found ")"
plugin.sp(29) : error 029: invalid expression, assumed zero
plugin.sp(29) : fatal error 190: too many error messages on one line

Compilation aborted.
5 Errors.